As the easily exploited hydrocarbon energy sources have been depleted, oil and gas wells have been drilled to ever deeper depths and have required more complex technology. Much of the current drilling activity is conducted from off-shore drilling platforms which often support twenty or more wells. All but one of the wells drilled from such a platform are necessarily deviated from the vertical axis.
Oil and gas wells are drilled into a reservoir of oil or gas wherein the reservoir generally consists of a porous rock which is filled with hydrocarbon liquids, hydrocarbon gases, water, and sometimes other liquids and gases. The pressure in the reservoir is considered "normal" when it is equal to the pressure exerted by a column of water extending from the surface to the reservoir depth. Petroleum reservoirs are often over-pressured below certain depths and can be under-pressured when depleted.
When a well is drilled into a reservoir, the reservoir fluids tend to flow into the wellbore and up to the surface unless the pressure exerted by the column of fluid in the wellbore exceeds the reservoir fluid pressure. Well bore fluid weight is, therefore, extremely important in well control. A "blowout" is defined as a fluid flow from the reservoir which is not under control--either to the surface or to another underground reservoir.
Wells are normally drilled with a liquid in the wellbore called "mud" which is composed of either a water or oil phase carrier and solid components to give the mud viscosity and extra weight or pressure. Blowouts generally occur when the mud weight is too low (below reservoir pressure) due most often to too low a solids content or dilution by produced liquids, notably gas, which lowers the mud weight. Gas dilution blowouts are generally the worst because of the extreme lowering pressure and fire hazards.
Offshore platform blowouts are much harder to control than land blowouts due to the logistics and personal danger. There are typically about 160 reported blowouts per year, most of which are controlled within a few days largely by natural processes such as bridging. About thirty percent are controlled by surface capping and typically within thirty days. About five blowouts per year require relief wells to control.
The term "relief well" is a historical term and is actually a misnomer when applied to modern kill wells today. Until about 12 years ago when search methods were developed, relief wells had a very small chance of intersecting the blowout. Consequently, the "relief method" was used to control blowout wells. The relief method involves the drilling of multiple producing wells in the vicinity of the blowout to allow the production from these wells to "relieve" the reservoir pressure. Hence the term relief well.
As was mentioned above, until recently relief wells had a very small chance of intersecting a blowout because of inadequate search methods. Search methods are heavily dependent on accurate surveys of the relief wellbore. Two angles are used to describe the direction of a well: (1) inclination (often called drift angle) is the angle between the borehole and the vertical axis which is defined by gravity; (2) azimuth is the horizontal directional component of the well which is measured clockwise from true geographic north. Directional drillers often refer to the azimuth as the direction and use a quadrant system of notation such as N85:30E or S80:00E. These two directions are mostly east and 141/2 degrees different. The equivalent azimuth statements are 85.5 and 100.0 degrees.
Wells which are deviated from the vertical axis are represented by maps or plots. There are two common views of a deviated well: (1) the plan or horizontal view which is a projection of the well path on the horizontal plane with North-South and East-West axis; and (2) the section view which is a projection of the well path of a vertical plane, usually a plane closest to the average horizontal direction of the well path. Deviated wells are also described by "build" and "drop" rates. The build and drop rates refer to the rate at which the inclination (or drift) is increased or decreased, respectively. The rates are normally quoted in degrees per hundred feet. Typical rates are 1-4 degrees per hundred feet. In addition, the rate of curvature of a deviated well is called "dogleg severity."
In the past, changes in azimuth or direction were not made except to "correct" the direction of a well which had deviated from the planned two dimensional course. Such corrections turn left or right and have the same rate restrictions as build or drop. Normally, build or drop corrections are not mixed with left and right corrections, but, are executed independently. Modern "bent housing" downhole motors make drilling in three dimensions more practical than drilling with the previous "bent sub" methods because of the greatly reduced length below the bend. Normal directional drilling is still basically two dimensional.
The surveying and drilling system provided by the present invention is fundamentally a three dimensional process which is extremely important for the drilling of relief wells. As will be discussed in greater detail below the invention planning system is capable of extreme precision in directing the relief well to an exact three dimensional target. The three dimensional quality generates less total curvature than previous surveying methods, thus representing a major improvement over the prior art. By contrast, state of the art directional drilling planning has previously been geared to hitting large targets usually greater than 100 feet across, which do not require precision planning.
Until approximately 1975, there were no surveying systems which were capable of providing an accurate quantitative measurement of the direction and distance to a blowout well from the well bore of the relief well. Until 1975, conventional wireline formation logging tools were used in relatively unsuccessful attempts to guide the relief well to the blowout well. The most successful systems used until that time were based on the Ulsel log, a long spaced resistivity log which was used in conjunction with special sonic detectors. The Ulsel log could be used to detect the blow out well casing, but provided a very poor range estimate and absolutely no directional information. Furthermore, the sonic detectors could detect the sound in the vicinity of high gas production and could detect the depth of the blowing formation, but provided very poor ranging and no directional information.
U.S. Pat. No. 4,072,200 issued Feb. 7, 1978, to Morris et al discloses a device for detecting the static magnetization of tubulars in a blowout well from a wireline tool in the relief well. This device has been used in approximately 90 previous cases wherein it was necessary to located a remote well. The device disclosed in the Morris patent, sometimes referred to as "MagRange.TM.", detects magnetic monopoles normally associated with tubular (either casing or drill collars) joints in the blowout wellbore. The occurrence and distribution of poles is virtually random, making the reliability of detection uncertain at a given joint and generally limited to the 30 or 40 foot joint spacing. The range from a joint is typically 25 feet but varies from virtually zero up to approximately 50 feet. The range from the end of the casing or drill pipe is much higher, on the order of 100 feet.
Another surveying technique, disclosed in U.S. Pat. No. 4,529,939 issued on July 16, 1985, to Kuckes, is based on an induction magnetic method. In the Kuckes method, alternating current (1 Hz) is injected into the earth from a wireline tool in the relief well. At the end of the wireline, typically 350 feet below the current injector, two vector magnetic sensors mounted mutually perpendicular to each other, and perpendicular to the borehole, synchronously (with the injected current) detect magnetic fields emanating from the blowout tubulars due to current having collected in the tubulars and flowing along the longitudinal axis of the respective tubulars. This method has a range of between 100 and 200 feet, depending on the resistivity of the formations. It also has an improved accuracy with respect to the determination of direction. The range estimate based on the Kuckes method has an approximate accuracy of between 20 and 50 percent, depending on the distance.
The two survey tools described above have significantly improved the art of drilling relief wells to intersect and kill a blowout well. Despite these advances, however, significant difficulties remain with respect to navigation of the relief wellbore. In particular, surveying error of only a fraction of a degree can result in significant deviations from the desired target at depths of two miles or more.
Numerous errors can seriously complicate efforts to kill a blowout well by drilling a relief well. In theory, the use of an off vertical relief well to intersect the blowout could be achieved accurately if the location of both the relief wellbore and the blowout wellbore could be known with sufficient accuracy. In practice however, the actual location of the blowout wellbore is rarely known with sufficient accuracy. Numerous errors are incorporated into the logging of the off vertical deviations during the drilling of the well. In general the types of errors which can be encountered with the location of the blowout wellbore are the following: 1) errors in the surface survey location; 2) random erros in the directional surveys; and 3) systematic errors in the directional surveys.
Various authors have previously recognized individual errors which might be encountered in determining the location of a wellbore. For example, in an article entitled "Borehole Position Uncertainty-Analysis of Measuring Methods and Derivation of Systematic Error Model," Journal of Petroleum Engineering and Technology, December 1981, pages 2339-50, Wolff and De Wardt, discuss systematic errors which are often incorporated into direction surveys of a wellbore. In addition, in another article, "Analysis of Uncertainty in Directional Drilling," Journal of Applied Petroleum April 1969, Walstrom, Brown and Harvey, discuss random errors which can significantly affect the accuracy of directional surveys of a wellbore. The errors described in the above mentioned articles apply to both the target blowout wellbore and to the relief wellbore. Although the above mentioned articles are useful to the extent they describe two types of errors which contribute to uncertainty as to the location of the respective wellbores, the art has heretofore lacked a teaching of a method for combining these uncertainties to provide a more effective surveying system for using relief wells to kill blowout wells. Furthermore, the prior art surveying techniques have failed to adequately incorporate errors related to the surface survey location. The infamous Ixtoc 1 is an example case where the error in the surface site location, later measured to be 224 feet, delayed the kill of the blowout by several months. The surface site error of the relief well is typically much smaller than that of the original blowout wellbore, principally due to greater care in documenting the location of the relief well.
In view of the foregoing discussion, it is evident that an accurate method for determining the relative locations of the original blowout wellbore and the relief wellbore is needed. More specifically, it is apparent that there is a need for a more effective surveying system which is capable of combining errors in the surface survey location with random errors and systematic errors related to directional surveys. The surveying system of the present invention, as described in greater detail below, provides a relative probable location distribution (RPLD) which includes an estimate of surface site errors and the systematic and random errors due to directional surveys of both the blowout and relief wells.